Tuesday, November 29, 2011

EIGRP Route Summarization

Summarizing routes automatically up to the major network boundary is an EIGRP feature that inherited from traditional distance vector routing protocols (the predecessor of EIGRP is IGRP, a DV routing protocol). Traditional DV routing protocols are classful routing protocols that must summarize at the network boundaries. They have no idea regarding the subnet masks for non-directly connected networks, as subnet masks are not being exchanged in routing updates.

EIGRP performs autosummarization by default. This feature can be disabled and must be disabled for discontiguous networks. Additionally, EIGRP summary routes within a network can be configured on any bit boundary as long as a more-specific route exists in the routing table. EIGRP supports summarization at any router; unlike OSPF, which must be performed on an Area Border Router (ABR) or Autonomous System Border Router (ASBR).

When route summarization is configured on an interface, a summary route destined to the Null0 interface, a directly connected, software-based interface as the next-hop interface will be added to the routing table. The Null0 interface is often used to prevent routing loop in a network. Ex: If the summarizing router receives a packet to an unknown subnet that is part of the summarized range, the packet matches the summary route based on the longest match. The packet is forwarded to the Null0 interface and discarded. This prevents the summarizing router from forwarding the packet to a default route which could possibly create a routing loop. The null route will only be seen on the summarizing router.

The ip summary-address eigrp {as-num} {network-address} {summary-mask} interface subcommand creates a summary route. The summary route is advertised only if an internal component route (a more-specific entry) of the summary route is exists in the routing table of the summarizing router [1]. The component route does not need to be a directly connected subnet; it may be learnt from other routers via a routing protocol.
[1] – This means that if all components that make up the summary disappear, or only external (redistributed) components exist, the summary route is not installed and advertised.

The metric of the summary route is selected based upon the lowest metric of the component routes.

IP EIGRP summary routes have an administrative distance value of 5. Administrative distance values are locally significant and therefore are not propagated to other routers; the neighbor routers which receive the summary route still have an Internal EIGRP route with an AD of 90. The EIGRP summary route with an AD of 5 will only be shown on the summarizing router with the show ip route {summary-route} EXEC command.

Network Setup for EIGRP Route Summarization

Autosummarization on has been disabled RT1 and RT2 with the no auto-summary router subcommand, therefore RT3’s routing table contains all the subnets on RT1 and RT2. Configuring a summary route on RT3 Fa0/0 consolidates the 10.1.1.0/24 and 10.1.2.0/24 routes and advertises a single 10.1.0.0/16 summary route to EXT.